Method of preparing β-dihydrothebaine

ABSTRACT

The reaction of thebaine or dihydrothebaine- phi  with potassium in liquid ammonia respectively in the absence or presence of catalyst yields  beta -dihydrothebaine.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Beta-dihydrothebaine has the structural formula, ##STR1## is a member ofthe morphine alkaloid family in general, and is a derivative of thebainein particular. Thebaine, a natural alkaloid, has the followingstructure: ##STR2## In formulas I and II, as elsewhere in thisspecification, a dashed line represents a covalent bond projecting belowthe plane of a reference atom while a wedged or heavily-accented linesignifies a covalent bond projecting above such plane. Bothβ-dihydrothebaine and thebaine are important intermediates in thesynthesis of analgesic narcotic antagonist compounds. For a thoroughreview of the field, refer to: Bentley, K. W., The Chemistry of theMorphine Alkaloids, Chapters XII and XV, London, Oxford, 1954; andBentley, K. W., "The Morphine Alkaloids", in: The Alkaloids, Vol. XIII,Manske, R. H. F. (Ed.), Chapter 1, New York, Academic Press, 1971.

THE PRIOR ART

Schmid and Karrer (Helv. Chim. Acta, 33.873 [ 1950]; hereafter "Schmid")report that reduction of thebaine with LiAlH₄ in benzene and etherresults in a 42% yield of β-dihydrothebaine.

Bentley, Lewis and Taylor (J. Chem. Soc., C, 1945 [1969]; hereafter"Bentley I"), however, find the Schmid reaction unsatisfactory: largeamounts of thebaine remain unreacted even after the reaction proceedsfor more than 48 hours; the process yields phenolic products other thanβ-dihydrothebaine; and the yield reported in Schmid is not reproducible.Bentley I further describes the results of studies in the reduction ofthebaine utilizing mixtures of LiAlH₄ and AlCl₃. Lithium aluminumhydride and aluminum chloride at respective ratios of 1:1, 1:3, or 1:4provide neodihydrothebaine, ##STR3## as the major product; and traces ofthebaine-A enol methyl ether, ##STR4## On the other hand, LiAlH₄ andAlCl₃ at respective ratios of 4:1 or 3:1 yield thebainone-A enol methylether, IV, as the principle product and traces of neohydrothebaine(III), β-dihydrothebaine (II), and an unidentified C-4 phenoliccompound.

Bentley, Robinson, and Wain (J. Chem. Soc., 958 [1952]; hereafter"Bentley II") elucidate the structure of dihydrothebaine-φ, ##STR5## andcommunicate that reaction of thebaine with sodium in liquid ammoniaresults in a 95% yield of dihydrothebaine-φ without trace of theβ-dihydrothebaine isomer.

Birch and Friton (Aust. J. Chem., 22:971 at 972 [1966]; hereafter"Birch") disclose that: "Reduction of thebaine with sodium in liquidammonia opens the ether ring, but the resulting methoxycyclohexadine[i.e., dihydrothebaine-φ] has unconjugated double bonds which cannot bereadily conjugated by the usual basic reagents. (emphasis supplied)."The Birch reference thus teaches that isomerization of dihydrothebaine-φ(V) to β-dihydrothebaine (I) will not proceed with alkali metal amidesin liquid ammonia.

Table A provides a synopsis of the preceding prior art. This analysis ofthe prior art thus indicates that there is no current simple,satisfactory method for producing β-dihydrothebaine in reasonably goodyields.

SUMMARY OF THE INVENTION

The subject matter of this reaction is a method of preparingβ-dihydrothebaine by reacting thebaine (II) with potassium in liquidammonia in the absence of catalysts or by reacting dihydrothebaine-φ (V)with potassium in the presence of catalyst. When thebaine is thestarting material, the method effects 95% conversion thereof and yieldsa 1:1 reaction mixture of β-dihydrothebaine and dihydrothebaine-φ. Whendihydrothebaine-φ is the starting compound, the method converts 50% ofthat compound to β-dihydrothebaine; no side reactions occur.

The method is straightforward. Thebaine (II) is added to a solution ofpotassium in an excess of liquid ammonia; 1 to 3 equivalents ofpotassium are utilized to 1 equivalent of starting material, although2.3 equivalents of the former are preferred. The reaction is allowed toproceed for a period of about 0.5 to 3 hours, which period represents aconvenient rather than a critical time. The reaction, of course,proceeds at liquid ammonia temperature ranges of approximately -78° C.to approximately -33° C. At the end of the reaction, unused potassium,if any, is removed by addition of a small quantity of any suitableorganic reagent, such as ethanol or diethyl ether. Beta-dihydrothebaineis easily isolated from the neutralized reaction mixture by a suitableorganic solvent for that compound. Such solvents are well-known in theart as disclosed in the cited references. Use of potassium withoutcatalysis (e.g. by ferric cation) in the method is a key factor.

                                      Table A                                     __________________________________________________________________________    Summary of Reactions Disclosed by Prior Art                                   Reference                                                                            Starting Material                                                                        Reactants       Product                                     __________________________________________________________________________    Schmid            LiAlH.sub.4                                                                              β-dihydrothebaine (42%)*                                      LiAlH.sub.4 -- AlCl.sub.3                                                                neodihydrothebaine ( III)                                          (1:1, 1:3, or 1:4)                                                                       and trace of thebainone-A                                                     enol methyl ether (IV)                           Bentley I                                                                            Thebaine (II)                                                                            LiAlH.sub.4 -- AlCl.sub.3                                                                neohydrothebaine ( III)                                            (4:1 or 3:1)                                                                             and traces of β-dihydro-                                                 thebaine (I)                                     Bentley II        Na -- liq NH.sub.3                                                                       dihydrothebaine-φ ( V)                       Birch  dihydrothebaine-φ                                                                    Na -- liq NH.sub.3                                                                       None                                                    ( V)                                                                   __________________________________________________________________________     *Disputed by Bentley I                                                   

Comparative studies of the reaction of thebaine with related elements,e.g., lithium, sodium, or calcium or with potassium catalyzed by ferricchloride show that the reaction yields dihydrothebaine-φ rather thanβ-dihydrothebaine. Example 3, below, provides details of comparativeexperiments involving the process; Table B summarizes the results ofthose experiments.

                  Table B                                                         ______________________________________                                        Reaction of Thebaine with Alkali Elements                                     in Liquid Ammonia                                                             Example                    Thebaine                                           No.    Element   Equivalents                                                                             Converted-%                                                                            % I  % V                                  ______________________________________                                         1     K         1.0        50*     50    50                                   1     K         2.3       95       50    50                                  3A     Ca        2.3        50*     0    100                                  3B     Li        2.3       62       0    100                                  3C     K/FeCl.sub.3                                                                            2.3       95       0    100                                  --     Na**      2.3       95       0    100                                  ______________________________________                                         *50% of unreacted thebaine recovered                                          **results reported in Bentley II                                         

Alternately, dihydrothebaine-φ (V) is added to a solution of potassiumin an excess of liquid ammonia in the presence of a catalyst such asFe(NO₃)₃ ·9H₂), FeCl.sub. 3, or similar catalysts known to the art: see,for example, Fieser L. F. and Fieser, M., Reagents for OrganicSynthesis, Vol. I, New York, J. Wiley & Sons, 1967. Three to fiveequivalents of potassium to 1 equivalent of dihydrothebaine-φ areutilized, although 3 equivalents of the former are preferred. Reactionparameters, such as temperature and time of reaction, and solvents forisolation of the product of the reaction, β-dihydrothebaine, areidentical to those described above. Details of the reaction are providedin Example 2, below.

The disclosed method, therefore, affords a convenient, reliable meansfor virtually quantitative conversion of thebaine or dihydrothebaine-φto β-dihydrothebaine in surprisingly high yields.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1 Reaction of Thebainewith Elemental Potassium in Liquid Ammonia in the Absence of Catalyst

The apparatus utilized in the following procedure was a one-liter,three-necked flask fitted with: a mechanical stirrer; a reflux condenserwhich in turn was adapted with a drying tube (KOH); a ground glassstopper; and a heating mantle for insulation. Approximately 600 ml ofliquid ammonia were introduced to the flask; 44.0 g (0.141 mol) ofthebaine were then added to the ammonia, resulting in a sand coloredmixture. While that mixture was being stirred, 12.6 g (0.322 mol; 2.3equivalents) of elemental potassium were added slowly (0.5 to 3 hours).As the potassium was being added, the resulting mixture first becameorange in color, which eventually turned dark red. The reaction mixturethen was stirred for 1 hour, the latter time being not critical butconvenient. The reaction was then quenched by addition of 24 ml ofabsolute ethanol resulting in a dark brown solution. The solution wasstirred for an additional 0.5 hour, and the ammonia was allowed toevaporate overnight. Neither the reaction time nor the method ofeliminating the liquid ammonia are critical to the process; the ammonia,for example, might have been driven off by controlled heating. Afterevaporation of the ammonia, 500 g of crushed ice and 150 ml of waterwere slowly added to the reaction mixture which then assumed a greencolor. The mixture was then treated with solid carbon dioxide whichprecipitated the reaction products from the mixture. Two liters of etherthen were added to the mixture; a very small interface was filtered bygravity, and the layers separated. The ether-layer was washed four timeswith 250 ml of water, dried (Na₂ SO₄), and concentrated to yield a tanfoam. The latter was dried under high vacuum, and 35.7 g of a tan powderwas obtained. Analysis of that product by nuclear magnetic resonance(NMR) revealed it to be a 1:1 mixture of β-dihydrothebaine (olefinicprotons: d, 1H, 5.73 ppm; and d, 1H, 4.80 ppm) and dihydrothebaine-φ(olefinic protons: s, 1H, 6.10 ppm; and t, 1H, 5.57 ppm). That mixturewas boiled in 250 ml of ligroin (b.p., 63°-75° C.), and then ethylacetate was added until the solution was complete. The solution wasfiltered while still hot and the filtrate was allowed to come to roomtemperature. Fifteen gm (24% overall yield) of β-dihydrothebaine (freeof dihydrothebaine-φ by NMR analysis) were thus obtained.

Utilization of 5.48 g (1 equivalent) of potassium in the above processprovided a 1:1 mixture of β-dihydrothebaine and dihydrothebaine-φ, butonly 50% of the thebaine was reduced.

EXAMPLE 2 Reaction of Dihydrothebaine-φ with Potassium in Liquid Ammoniain the Presence of Catalyst

Apparatus utilized in this example was similar to that described inpreceding Example 1 except that a 100 ml flask was used. Approximately65 ml of liquid ammonia and a catalytic amount of Fe(NO₃)₃ ·9H₂ O wereadded to the flask followed by the slow addition of 750 mg (19.2 mmol; 3equivalents) of potassium: the resulting solution, which was steel-greyin color, was then stirred for approximately 0.5 hour; the latter periodof time was not critical but rather convenient. Addition of 2.0 g (6.4mmol) of dihydrothebaine-φ turned the reaction mixture to a red colorwhich was indicative of presence of the di-anion. The reaction mixturewas then stirred for approximately 2 hours, a convenient rather thancritical period of time. Addition of 10 ml of diethyl ether quenched thereaction. Ten ml of 20% aqueous diethyl ether solution was then added.The ammonia was evaporated under a fume hood. An additional quantity ofaqueous diethylether and an excess of solid ammonium chloride were addedto the reaction mixture. Aqueous and ethereal layers were separated. Theaqueous phase was extracted once with diethyl ether. Ethereal phaseswere combined, washed twice with water, and dried over anhydrous sodiumsulfate. Concentration under reduced pressure left 1.58 g (79% yield) ofa red resin, the NMR spectrum of which indicated a 1:1 mixture ofβ-dihydrothebaine and dihydrothebaine-φ.

EXAMPLE 3 Comparative Reactions of Thebaine in Liquid AmmoniaRespectively with: (a) Calcium; (b) Lithium; and (c) Potassium Catalyzedby the Presence of Ferric Cation

In the succeeding examples, the apparatus utilized was that described inExample 1.

A. reaction of Thebaine with Calcium in Liquid Ammonia.

Approximately 250 ml of liquid ammonia was added to the reaction flaskcontaining 22.0 g (0.01 mol) of thebaine. To that mixture 3.2 g (0.081mol; 2.3 equivalent) of elemental calcium were added slowly. Thereaction mixture turned a red color, and was stirred for 12/3 hours.Eight ml of absolute ethanol were than added, and the reaction mixturewas then poured into a 1-liter beaker containing 500 g of crushed ice.The reaction mixture was then acidified by addition of solid carbondioxide and subsequently extracted with 1800 ml of ether and 400 ml ofchloroform. The combined organic solvent layer was washed with water anddried over anhydrous sodium sulfate. Filtration and concentrationyielded 19.2 g of a reddish solid. NMR analysis revealed a 1:1 mixtureof unreacted thebaine and dihydrothebaine-φ. The solid was extractedwith ether and only the dihydrothebaine-φ dissolved. In this manner a50% yield of dihydrothebaine-φ and a 50% recovery of unreacted thebainewas obtained. No β-dihydrothebaine was produced.

B. reaction of Thebaine with Lithium in Liquid Ammonia.

Forty-four g (0.141 mol) of thebaine and 500 ml of anhydrous liquidammonia were independently added to the reaction flask, giving atan-colored mixture. After slow addition of 2.2 g (0.32 mol; 2.3equivalent) of metallic lithium, the reaction mixture was stirred for1.5 hours, and thereafter the reaction was quenched with 5 ml ofabsolute ethanol. After evaporation of the ammonia, the remainingresidue was dissolved in 500 ml of water acidified with solid carbondioxide. A solid precipitated which was dissolved in ether. The etherextract was washed with water, dried (anhydrous sodium sulfate),filtered, and concentrated, yielding 27.2 g (0.087 mol; 62% yield) ofdihydrothebaine-φ, the identity of which was confirmed by NMR. Noβ-dihydrothebaine was produced.

C. reaction of Thebaine with Potassium in Liquid Ammonia Catalyzed byFerric Chloride.

Twenty-two g of thebaine (0.07 mol) was added to the reaction flaskwhich was cooled to -78° C; 250 ml of liquid ammonia and some crystalsof ferric chloride were then added to the flask, followed by 6.3 g(0.161 mol; 2.3 equivalent) of potassium. That reaction mixture wasstirred for 1 hour and assumed an orange color. Addition of 8 ml ofabsolute ethanol quenched the reaction. After evaporation of theammonia; 500 ml of water was added to the reaction mixture followed bysolid carbon dioxide. Subsequent extraction with ether, washing withwater, drying, filtration and concentration gave a 90% yield ofdihydrothebaine-φ, verified as such by NMR analysis. No dihydrothebainewas produced.

What is claimed is:
 1. A method of producing β-dihydrothebaine, whichmethod comprises:the step of reducing thebaine with from 1 to 3equivalents of potassium in liquid ammonia without any catalyst.
 2. Themethod as in claim 1 utilizing 2.3 equivalents of potassium.